The invention includes novel chemical compounds having specific binding in a biological system and capable of being used for positron emission tomography (PET) and single photon emission (SPECT) imaging methods.
The ability of analog compounds to bind to localized ligands within the body would make it possible, in principle, to utilize such compounds for in situ imaging of the ligands by PET, SPECT and similar imaging methods. In principle, nothing need be known about the nature of the ligand, as long as binding occurs, and such binding is specific for a class of cells, organs, tissues or receptors of interest. PET imaging is accomplished with the aid of tracer compounds labeled with a positron-emitting isotope (Goodman, M. M. Clinical Positron Emission Tomography, Mosby Yearbook, 1992, K. F. Hubner et al., Chapter 14). For most biological materials, suitable isotopes are few. The carbon isotope, [.sup.11 C], has been used for PET, but its short half-life of 20.5 minutes limits its usefulness to compounds that can be synthesized and purified quickly, and to facilities that are proximate to a cyclotron where the precursor [.sup.11 C] starting material is generated. Other isotopes have even shorter half-lives. [.sup.13 N] has a half-life of 10 minutes and [.sup.15 O] has an even shorter half-life of 2 minutes. The emissions of both are more energetic than those of [.sup.11 C]. Nevertheless, PET studies have been carried out with these isotopes (Hubner, K. F., in Clinical Positron Emission Tomography, Mosby Year Book, 1992, K. F. Hubner, et al., Chapter 2) A more useful isotope, [.sup.18 F], has a half-life of 110 minutes. This allows sufficient time for incorporation into a radio-labeled tracer, for purification and for administration into a human or animal subject. In addition, facilities more remote from a cyclotron, up to about a 200 mile radius, can make use of [.sup.18 F] labeled compounds. Disadvantages of [.sup.18 F] are the relative scarcity of fluorinated analogs that have functional equivalence to naturally-occurring biological materials, and the difficulty of designing methods of synthesis that efficiently utilize the starting material generated in the cyclotron. Such starting material can be either fluoride ion or fluorine gas. In the latter case only one fluorine atom of the bimolecular gas is actually a radionuclide, so the gas is designated .sup.18 F-F. Reactions using .sup.18 F-F as starting material therefore yield products having only one half the radionuclide abundance of reactions utilizing K.sup.18 F as starting material. On the other hand, [.sup.18 F] can be prepared in curie quantities as fluoride ion for incorporation into a radiopharmaceutical compound in high specific activity, theoretically 1.7 Ci/nmol using carrier-free nucleophilic substitution reactions. The energy emission of [.sup.18 F] is 0.635 MeV, resulting in a relatively short, 2.4 mm average positron range in tissue, permitting high resolution PET images.
SPECT imaging employs isotope tracers that emit high energy photons (.gamma.-emitters). The range of useful isotopes is greater than for PET, but SPECT provides lower three-dimensional resolution. Nevertheless, SPECT is widely used to obtain clinically significant information about analog binding, localization and clearance rates. A useful isotope for SPECT imaging is [.sup.123 I ], a .gamma.-emitter with a 13.3 hour half life. Compounds labeled with [.sup.123 I] can be shipped up to about 1000 miles from the manufacturing site, or the isotope itself can be transported for on-site synthesis. Eighty-five percent of the isotope's emissions are 159 KeV photons, which is readily measured by SPECT instrumentation currently in use.
Use of [.sup.18 F] labeled compounds in PET has been limited to a few analog compounds. Most notably, [.sup.18 F]-fluorodeoxyglucose has been widely used in studies of glucose metabolism and localization of glucose uptake associated with brain activity. [.sup.18 F]-L-fluorodopa and other dopamine receptor analogs have also been used in mapping dopamine receptor distribution.
Other halogen isotopes can serve for PET or SPECT imaging, or for conventional tracer labeling. These include .sup.75 Br, .sup.76 Br, .sup.77 Br and .sup.82 Br as having usable half-lives and emission characteristics. In general, the chemical means exist to substitute any halogen moiety for the described isotopes. Therefore, the biochemical or physiological activities of any halogenated homolog of the described compounds are now available for use by those skilled in the art, including stable isotope halogen homologs. Astatine can be substituted for other halogen isotopes, [.sup.210 At] emits alpha particles with a half-life of 8.3 h. At-substituted compounds are therefore useful for tumor therapy, where binding is sufficiently tumor-specific.
Two classes of benzodiazapine receptors are known, having different locations and binding characteristics. In the central nervous system, particularly cerebral cortex, are the "BZ" receptors which are part of a complex that includes the GABA.sub.A receptor and a chloride ion channel and which functions in GABAA mediated neurotransmission. Classical benzodiazapines such as diazepain, act as agonists, having axiolytic, anti-convulsant, myorelaxant and amnesia-producing effects. Aumazenil is a specific BZ receptor binding compound which acts as an antagonist, having no direct effect but blocking the effects of agonists and inverse-agonists.
A second class of receptors (PK) are widespread throughout the human body, being especially prevalent on macrophages and glial cells. The PK receptors are found to be concentrated at sites of injury or inflammation. Specific tissue damage can be localized by observing a high concentration of PK receptors, in particular, at sites of brain injury, premorbid atherosclerotic plaque and neuronal injury in patients with intracranial tumor, multiple sclerosis and infarcts. The relative binding affinities of various benzodiazapines differ in BZ and PK receptors. Certain isoquinoline carboxamides have been shown to be potent and selective antagonists at the PK binding site. (Dubroeucq et al. French Patent No. 8,207,217. For a general review see Pike, V. W., et al. (1993) Nucl. Med. Biol. 2:503-525.) The compound PK11195 [1-(2-chlorophenyl)-N-methyl-N-(alkyl)-3-isoquinoline carboxamide] binds strongly to PK receptor sites.
PK receptor site distribution can be imaged using PET or SPECT, as shown by studies using [.sup.11 C]-labeled PK11195. [Charbonneau, P. et al. (1986) Circulation 73:476-483; Hashimoto, K. et al. (1989) Ann. Nucl. Med. 3:63-71. The potential clinical utility of PK11195 cannot be realized using ["C] label, as discussed.
A [.sup.18 F]-labeled analog, PK14105 has been described by Pascali et al. (1990) Appl. Radiat. Isot. 41:477-482, PK14105 is an analog of PK11195 with the C2 moiety replaced by [.sup.18 F] and a NO.sub.2 group in sara orientation to the fluoro-group. The compound was labeled by nucleophilic substitution of the chloro analog by no-carrier-added (NCA) [.sup.18 F] produced in a cyclotron. However, the procedure required a tedious double pass through HPLC to remove the pharmacologically active chloro precursor. The purified product had a specific radioactivity of 0.4 Ci/Mmol. [For a general review of radioligands for both central and peripheral benzodiazapine receptors, see Pike, V. W., et al. (1993) Nucl. Med. Biol. 3:503-525].
Other substituted isoquinoline and quinoline carboxamides have been disclosed, some with halogen substituents. The compounds disclosed to date have been synthesized by processes that introduce the halogen at an early step in the process, which results in substantial loss of usable half-life because of the short half-lives of the nuclides most useful for PET and SPECT analysis. Notable examples include Dubroeucq, et al. U.S. Pat. No. 4,499,094 in which a halogen can exist as a substitutent of a phenyl group attached to the isoquinoline ring. European patent EPO B 0,210,084 describes substituted quinoline and isoquinoline carboxamides that can have halogen substituted on the non-heterocyclic ring, as well as on a phenyl attached to the heterocyclic ring. Synthesis was based on introducing the halogen at an early step or by using a halogenated starting material. Mendes, et al. U.S. Pat. No. 5,026,711 described quinoline carboxamide derivatives having the possibility of halogen substituted on the non-heterocyclic ring. Dubroeucq, et al. U.S. Pat. No. 4,684,652 described quinoline derivatives and isoquinolines (including carboxamides) having halogen substituted adjacent to the heterocyclic N or on the non-heterocyclic ring. The compounds were primarily specific for binding cerebral diazopine (BZ) receptors. Halogen substituted compounds were synthesized by a process that included introducing the halogen at an early stage of synthesis, by using a halogenated starting material. Costa, et al. U.S. Pat. No. 5,206,382 described substituted indoleacetamides having PK receptor binding activity. Halophenyl substituents were described. Synthesis was carried out by what may be deemed conventional early step halogen introduction. 1-oxo-quinoline derivatives having PK receptor binding activity have been described by Frost et al. U.S. Pat. No. 5,212,181.